The invention relates generally to the reduction of nitrogen oxides from flue gas, and, more particularly, to a method and apparatus for reducing ammonia slip to the atmosphere through adsorption/desorption at the air preheater, while simultaneously relieving the air heater pluggage due to ammonia sulfate and bisulfate formation and, further, to reduce the amount of ammonia attaching to the flyash collected by precipitators or baghouses.
Much of the electrical power used in homes and businesses throughout the world is produced in power plants that burn a fossil fuel (i.e. coal, oil, or gas) in a boiler. The resulting hot exhaust gas (also sometimes termed xe2x80x9cflue gasxe2x80x9d) turns a gas turbine or boils water to produce steam, which turns a steam turbine, and the turbine cooperates with a generator to produce electrical power. The flue gas stream is passed through an air preheater, such as a rotating wheel heat exchanger, that transfers heat from the flue gas to an incoming air stream, that thereafter flows to the combustor. The partially cooled flue gas is directed from the air preheater to the exhaust stack.
An important consideration for modem power plants is the cleanup of the exhaust gas. The exhaust gas produced in the boiler contains gaseous pollutants such as nitrogen oxides (xe2x80x9cNOxxe2x80x9d) and sulfur oxides (xe2x80x9cSOxxe2x80x9d), as well as particulates termed xe2x80x9cfly ashxe2x80x9d. Environmental laws establish permissible levels of gaseous pollutants and particulates that may be emitted from the exhaust stack of the plant. Various types of pollution control equipment are available to reduce the levels of gaseous pollutants and particulates from the flue gas before it reaches the exhaust stack. For example, among other methods, NOx is often removed by selective catalytic reduction (SCR) and/or selective non-catalytic reduction (SNCR), and fly ash is often removed by an electrostatic precipitator (ESP) and/or a baghouse. The invention herein deals with those particular pollution control systems which utilize ammonia within the process in order to initiate, cause and/or supplement the removal of NOx, and in particular SCR, SNCR and/or staged systems (i.e. systems which include one or more SCR or SNCR systems).
To remove the NOx, a nitrogenous compound, such as ammonia, is injected into the flue gas stream. The ammonia reacts with the NOx to form nitrogen and water, reducing the NOx content of the flue gas. The reaction of ammonia and NOx may be performed at high temperature without a catalyst, a process termed xe2x80x9cselective non-catalytic reductionxe2x80x9d (SNCR), or at lower temperature in the presence of a catalyst, a process termed xe2x80x9cselective catalytic reductionxe2x80x9d (SCR).
SNCR is accomplished by injecting a nitrogenous compound, such as a source of ammonia, into the hot flue gas, and permitting the reduction reaction to occur in the flue gas. U.S. Pat. Nos. 3,900,554, 4,208,386, and 4,325,924 illustrate known types of SNCR applications. SCR is generally accomplished at lower temperatures than SNCR, and necessitates the use of a catalyst, which is placed onto surfaces of catalyst modules, which are positioned within a flue gas stream. U.S. Pat. No. 5,104,629 illustrates one known type of SCR installations.
It is important to accomplish the reaction of the ammonia and NOx in an efficient manner, for maximum possible reaction of both the NOx and the ammonia. If the reaction is incomplete, either NOx or ammonia (or both) may pass through to the stack and be emitted to the atmosphere. Both NOx and ammonia are classified as pollutants, and their emission is to be maintained within legal limits. Furthermore, depending upon the temperature at the cold end of the air preheater, excess ammonia slip may cause clogging of the space between adjacent air preheater heating elements because of the formation of ammonium sulfate/bisulfate, and/or agglomerated flyash. In addition, many power plants dispose of the collected flyash by selling it to purchasers who further process the flyash for commercial uses (i.e. lightweight aggregate for concrete mixtures). If the ammonia amount of which adheres to the flyash is relatively high (i.e. in excess of 100 ppm, by weight, or as otherwise mandated by users), the flyash may not be able to be sold, and the utility will have to pay for the disposal.
The prior art is replete with ways of preventing or alleviating ammonia slip (i.e. larger SCR units, more responsive control arrangements, early replacement of catalyst, and the like); however, in all instances, some ammonia slip will indeed enter the air preheater. It is specifically to the ammonia slip which enters the air preheater for which the instant invention is directed. In this regard, at the air preheater, very little original thinking has been directed for the last several years, for example: efforts to control or alleviate the effects of ammonium sulfate/bisulfate formation by applying an enamel coating to the air preheater elements, coupled with aggressive sootblowing (an expensive and short lived solution); and efforts to alleviate the ammonia slip by catalyzing the air preheater elements, such as is illustrated in U.S. Pat. Nos. 4,602,673 and 4,867,953 (a concept which to date has not proved economical or practical).
As will be discussed hereinafter in detail, the present invention overcomes, or in the least, greatly alleviates the prior art deficiencies discussed above, in an efficient, cost effective and novel manner.
The present invention includes an adsorbent compound which is integrally formed with, or adhered to, the exposed surfaces of some of the air preheater heating elements. The adsorbent is selected and structured to act as a molecular sieve to adsorb, or capture, molecular ammonia on the gas side of the air preheater, and to desorb, or release, such captured ammonia on the air side of the preheater. The desorbed ammonia is either destroyed in the boiler fire ball or, in the alternative, is used to destroy NOx in an SNCR and/or SCR stage of the NOx reduction system. The preferred pore size of the adsorbent material will be sufficient to adsorb ammonia (i.e 2.8 xc3x85), but less than 4 xc3x85, so as to prevent SO2 or SO3 penetration. As will be discussed hereinafter in the detailed description of the preferred embodiment, other critical criteria for the application of the existing invention is the selection of preheater layers or partial layers on which elements having adsorbent surfaces will be positioned, as well as the minimum catalytic activity of such adsorbent (i.e. to alleviate the concern for amplifying the potential of ammonium sulfate/bisulfate problems, or creating additional SO3 where perhaps none is required).
In view of the above, it is to be appreciated that an object and advantage of the present invention is to provide a method and apparatus for alleviating the problems of ammonia slip to the environment, by including a means and method for adsorbing excess system ammonia at the gas side of the air preheater and continually releasing an equilibrium portion of such adsorbed ammonia as the heating elements rotate to the air side of the preheater.
It is another object and advantage of the present invention to lessen air preheater element fouling due to ammonium sulfate/bisulfate deposits.
It is an additional object and advantage of the present invention to address the ammonia slip problem, without catalytic activity occurring in the area of adsorption/desorption.
It is still a further object and advantage of the invention as submitted herewith, to use the inventive concepts discussed in other fossil fuel burning systems for producing electrical power and which have ammonia slip from NOx reduction means and includes a rotary regenerative apparatus; for example, but without limitation, in conjunction with: known types of low dust SCR arrangements, also known as xe2x80x9ctail end typesxe2x80x9d, which include a gas to gas rotary regenerative heat uses which exchanges heat from the outlet of the gas flowing from an SCR reactor, to heat the incoming gas flow, which has exited from an FGD system and is being directed to such SCR reactor; and in gas turbine regenerative cycles wherein a regenerative heat exchanger uses heat from the gas flow leaving the SCR unit to heat the high pressure system air exiting from the compressor portion of the cycle, such air being subsequently additionally heated and directed to a turbine portion of the cycle.
These and other objects and advantages of the present invention will become more readily apparent upon a reviewing and reading of the following drawings and description in which: